Anisotropic conductive films have been so far used mainly in electrical connection of liquid crystal-driving IC (electrical connection will be hereinafter referred to merely "connection").
Many anisotropic conductive films have been disclosed so far. For example, JP-A-7-197001 and JP-A-4-242001 disclose anisotropic conductive films using conductive particles composed of metal-plated resin spheres. Furthermore, JP-A-61-55809, JP-A-5-40402, JP-A-7-73740 and JP-A-7-65028, for example, disclose anisotropic conductive films using metal powders such as nickel powders, solder powders, gold-plated nickel powders, etc.
Anisotropic conductive films are films comprising conductive particles dispersed in an organic binder. The anisotropic conductive film is stuck in advance to electrodes or terminals on a substrate to be connected, and then a corresponding further connection substrate or LSI to be connected is placed thereon, followed by pressing with heating, thereby drying or curing the organic binder. The conductive particles existing between the electrodes are deformed, whereby the resulting film has a high conductivity only in the thickness direction between the electrodes, while being insulative between the adjacent electrodes themselves. Thus, the anisotropic conductive films have been used in TAB (tape automated bonding) connection of IC for driving the panels of liquid crystal, plasma display, EL element, etc., bare chip connection of LSI, panel connection of flexible substrate, etc.
Conductive particles for use in the anisotropic conductive film include, for example, metal powders, metal-plated resin powders, etc. Metal powders include, for example, solder powders, nickel powders, gold-plated nickel powders, copper powders, silver powders, etc. Nickel powders have a high resistivity, a low circumferential resistance, and also the problem of increased connection resistance during use. Furthermore, nickel powders have a high hardness and require a rather high pressing force at the connection, thereby increasing substrate damage. For example, nickel powders, when used for connection of glass substrate, have such a problem as substrate breaking.
Solder powders cannot be used because of the high resistivity among the metal powders, and also have such a problem as frequent occurrence of half-molten state of solder, when heated for connection, because of the low melting point.
Gold-plated powders have such a problem as peeling of gold plating, when pressed, and also require a higher pressing force because of use of nickel. Thus, use of soft electrodes, for example, of copper often causes deformation of not only nickel powders but also electrodes.
Copper powders have such problems as poor reliability, because of easy occurrence of deterioration due to oxidation and of a restricted amount of conductive particles interposed between the electrodes.
Silver powders have such problems as easy deterioration of insulation resistance between the adjacent electrodes at a high humidity due to their migration and failure to apply to fine pitch connection.
Metal-plated resin articles, on the other hand, have a low conductivity per se and must be added in a large amount. In case of the metal-plated resin particles, the resin particles per se are easily cracked when the conductive particles are deformed by pressing, causing peeling of platings to lead to connection failure. Particularly in case of the heat cycle, voids are liable to form at the boundaries between the resin particles and the metal layers plated thereon due to a difference in thermal expansion coefficient therebetween, thereby causing the metal platings to peel away from the resin particles. JP-A-7-118618 discloses that, when conductive particles prepared by further providing a insulating coat of organic polymer on the surfaces of metal plated resin particles are used at the connection by pressing or with heating, the insulating coat on the surfaces of conductive particles existing between the electrodes will be broken to ensure interelectrode continuity, whereas the conductive particles failing to contribute to the connection still hold the insulating coat thereon to ensure an insulativeness between the adjacent electrodes. In that case, the insulating coat only on the conductive particles contributing to the connection is not always broken, but the insulating coat on the conductive particles failing to contribute to the connection is also damaged by heat transfer or by flow phenomena at the heating, thereby bringing about such a problem as a failure to maintain the insulativeness fully.
To enhance the productivity, it is required to conduct the pressing for such a short time as a few seconds at the film preparation. The anisotropic conductive composition containing the above-mentioned powders or particles had a poor heat conductivity, and thus heat setting of the composition by pressing for such a few seconds was not satisfactory and considerable connection failures took place. In fine pitch connection, dispersion of conductive particles throughout the fine pitch was also not satisfactory and a problem of connection failures was often encountered.
To produce an anisotropic conductive composition or film, it is necessary to attain satisfactory dispersion of conductive particles. Even in case of occurrence of slight coagulation of conductive particles or segregation of organic binder, etc. in the film, the amount of conductive particles interposed between the electrodes to contribute to the connection is reduced, thereby making them more liable to cause connection failures. Thus, the dispersibility of conductive particles is an important factor in the preparation of an anisotropic conductive composition or film. In the case of well known conductive particles such as metal-plated resin powders, etc., the plating layer will be often peeled away due to mechanical factors when stirred to obtain a high dispersibility.
In the case of nickel powders, a nickel oxide layer is formed on the powder surfaces and thus the dispersibility in an organic binder will be relatively high. However, the nickel oxide on the surfaces is an insulator and is also too hard to break simply by deformation, with the result that the nickel oxide layer remains still between the electrodes and nickel powders, failing to satisfy the connection conductivity.
In case of metal-plated powders such as, for example, gold-plated nickel powders, the gold plating layer will be peeled away when stirred to obtain a high dispersibility, bringing about such a problem as a failure to obtain satisfactory conductive characteristics.
Silver-copper alloy powders already disclosed by the present inventors can indeed give per se a satisfactory conductivity, but have such a problem as fluctuations in conductivity between the electrode terminals due to a poor dispersibility when used in the fine pitch connection.
In the connection of an anisotropic conductive film by pressing together with heating, it is necessary to cure the organic binder in the anisotropic conductive film at the same time, but such a short time as about 10 to about 20 seconds is required for the heating. To cure the organic binder containing conductive particles as dispersed therein for such a short time, the conductive particles must have an enhanced heat conductivity, but well known conductive particles have too low a heat conductivity to obtain a satisfactory curing.
As to the particle size distribution of conductive particles, JP-B-6-97573 discloses use of conductive particles having a ratio of minimum size to maximum size of 0.5-1 for the particle size distribution. However, the fine pitch connection mostly requires a film thickness of about 10 microns for the anisotropic conductive film, where the conductive particles having only the sharp particle size distribution hardly can give a uniform film, because a paste comprising the conductive particles, the organic binder and, if required, a solvent has no adequate thixotropy for the coating step in the film formation.
Thermoplastic resins and thermosetting resin have been so far used as an organic binder. In the case of using epoxy resin as a thermosetting resin, it is known to use a latent curing agent. Needless to say, the stability of dispersed curing agent is important for obtaining a preservation stability of anisotropic conductive films. Since the conventional conductive particles had no satisfactory dispersibility, an excessively strict strength and time were required for mechanical dispersion treatment of conductive particles for use in fine pitch connection. Thus, there were such problems as breaking of the protective film of microcapsule-type curing agent and consequent instability of the curing agent per se. For formation of anisotropic conductive films, it is necessary to use conductive particles having exceptional dispersibility and which causes no damage to the protective film of the curing agent during the coating process. For example, nickel powders in an indeterminate form, etc. have a poor dispersibility and a high hardness and thus have such problems as the necessity for application of pressure at the pressing and also breaking of the protective film of dispersed microcapsule-type curing agent during the process of forming anisotropic conductive films. Thus, the preservation stability of anisotropic conductive films is highly deteriorated and fluctuations in the connection conductivity are liable to occur.